Nocturnal Elevated Convective Storm Initiation: PECAN 4 July Hailstorm

During the Plains Elevated Convection at Night (PECAN) Experiment an isolated hailstorm developed on the western side of the PECAN study area on the night of 3-4 July 2015. One of the objectives of PECAN was to advance knowledge of the processes and conditions leading to pristine nocturnal convection initiation (CI). This nocturnal case developed more than 160 km from any other convective storms and in the absence of any surface fronts or bores. The case was unique in it was the only case during PECAN where forecasts successfully called for pristine nocturnal elevated convection initiation allowing for the deployment of many PECAN facilities in desirable locations.

The storm initiated within 110 km of S-Pol, directly over a scanning Doppler Lidar, within 25 km of the University of Wyoming King Air flight track, within a network of nine sounding sites taking 2-hourly soundings, and near a mobile mesonet track.

Importantly, even beyond 100 km in range, S-Pol was able to observe the pre-convection initiation cloud that aligned well with the satellite inferred cloud image, and was able to provide temporal evolution information on the cloud growth. The multiple observations of cloud base, stability and direct updraft observations were able to determine that the updraft roots were elevated, that CI was in general inhibited by the entrainment of very dry air just above cloud base and that a mesoscale mid-level wind-shift line was not influential in causing CI. Gravity waves were apparent in many of the observations and likely were the trigger for CI.